quick question on a fan and spray booth.

[werdy666]

If i have a 10inch fan, and 4 inch ducting, am i able to make something like a cone going from the fan down to the 4inches for the ducting? something like a cone for 10cm without having back pressure from the fan?

I understand i will lose some pressure from the fan, but previous attempts, not the cone idea, have caused air to be pushed back into the spray booth.

or am i stuck with 10inch ducting?

Thanks

Werdy666

[Veyron]

You might find these tips useful.

http://modelpaint.tripod.com/booth2.htm

[Adam Baker]

When I did my paint booth, the outlet on my fan was 9.25x4.5, and I created a converter to go to 4" ducting. I used a piece of metal that i found in the hvac section at Lowes. I'll post a couple pictures when I get home from work tonight of what i did.

[CFarias]

Ensure that your reducer is no less than 12 inches long and this should prevent undo amounts of backpressure.

Is your 4 inch line flex duct. Long runs of flex duct can build up a great deal of friction in the airflow, enough friction to overwhelm the power of a 10 inch fan. Also make sure that any bends in your 4 inch line is no tighter than the duct width. Make an 8 inch diameter circle out of cardboard and place this on the inside of any bends in your 4 inch line. If it does not fit then the bend is too tight creating more backpressure.

[CFarias]

Ensure that your reducer is no less than 12 inches long and this should prevent undo amounts of backpressure.

Is your 4 inch line flex duct. Long runs of flex duct can build up a great deal of friction in the airflow, enough friction to overwhelm the power of a 10 inch fan. Also make sure that any bends in your 4 inch line is no tighter than the duct width. Make an 8 inch diameter circle out of cardboard and place this on the inside of any bends in your 4 inch line. If it does not fit then the bend is too tight creating more backpressure.

[werdy666]

Originally Posted by CFarias

Ensure that your reducer is no less than 12 inches long and this should prevent undo amounts of backpressure.

Is your 4 inch line flex duct. Long runs of flex duct can build up a great deal of friction in the airflow, enough friction to overwhelm the power of a 10 inch fan. Also make sure that any bends in your 4 inch line is no tighter than the duct width. Make an 8 inch diameter circle out of cardboard and place this on the inside of any bends in your 4 inch line. If it does not fit then the bend is too tight creating more backpressure.

My ducting doesn't have to be long at all, in fact i think from the back of the booth it would be less than 12 inches...

I might have to rearrange my workstation to accomodate the 12inches required to reduce the 8 inches to 4.

Thanks CFarias

[Klaus Raddatz]

If i have a 10inch fan, and 4 inch ducting, am i able to make something like a cone going from the fan down to the 4inches for the ducting? something like a cone for 10cm without having back pressure from the fan?

I understand i will lose some pressure from the fan, but previous attempts, not the cone idea, have caused air to be pushed back into the spray booth.

or am i stuck with 10inch ducting?

Unfortunately, any time you decrease the size of the ducts, you increase the static pressure of that duct and reduce the airflow.

If the 10" fan has to work, try various HVAC duct parts at the local home improvement store to piece together a system of multiple 4" ducts. FYI, you would need at least six 4" ducts to equal the same area as one 10" duct, which means some fabrication is in order. Could be a lot of work. Do you know what the specs for the fan are? If not, I would find something with published CFM and SP spec's before going any further.

Hope this helps,
Klaus

[CFarias]

Klaus brings up some additional good points. Check the cfm and static pressure rating for the fan. Usually manufacturers will post this on their webistes.

However if you have a short run of duct as you say then its is not likely that the ductwork will cause problems unless its pinched or obstructed somehow. It might be possible that the vent hole for your duct is on an exterior wall that faces your area's prevailing wind. For example, if the pw in your hometown is south to north and your vent hole is on a south-facing wall, then the outside wind may be pushing into the duct and into the room. Your fan would not only have to fight any natural frictions within your system but it would also have to fight off the outside air trying to come inside. If it can't then you get no exhaust from the booth. A cover over the vent hole will mitigate this somewhat.